In a large skillet, brown meat in oil on all sides. Transfer meat and drippings to a 5-qt. slow cooker. Combine the celery, onion, soy sauce, sugar, garlic, ginger and pepper; pour over meat. Cover and cook on low for 5-1/2 to 6 hours or until meat is tender. , Add the green peppers, tomato sauce, bean sprouts, water chestnuts and mushrooms; cover and cook on low 1 hour longer. , Combine cornstarch and water until smooth; stir into beef mixture. Cover and cook on high for 30 minutes or until sauce is thickened. Serve with rice; if desired, sprinkle with chives. ,